Recently, development has been conducted for further improving images of digital cameras. In order to make it easy to view images of digital cameras, a process for emphasizing an edge in a taken image is performed.
FIG. 1 illustrates an existing edge emphasis technology. RGB signals are subjected to a color correction process at a color correction unit 10. The RGB signals that have been subjected to the color correction process are subjected to a gamma correction process at a gamma correction unit 11. The RGB signals that have been subjected to the gamma correction process are converted into YCbCr signals at an YC conversion unit 12. This process is referred to as “YC conversion.” A signal Y of the YCbCr signals is a luminance signal, and a signal Cb and a signal Cr of the YCbCr signals are color-difference signals. Edge detection is performed at an edge detection unit 13 for the luminance signal Y obtained after the YC conversion. After a detected edge component E is adjusted at an edge adjustment unit 14, an edge component Ead is added to a luminance signal Yn that has been noise-removed at a noise removal unit 15, thereby performing edge emphasis.
In the existing technology in FIG. 1, an edge is detected by using the luminance Y that is generated by performing YC conversion of RGB signals (Rc, Gc, and Bc) that have been subjected to a color correction process. Rc, Gc, and Bc are referred to as color-corrected RGB signals. In the color correction process, the following matrix operation is performed in order to match the color space of a camera with the color space of an output apparatus.
      (                            Rc                                      Gc                                      Bc                      )    =            (                                                  CC              ⁢                                                          ⁢              00                                                          CC              ⁢                                                          ⁢              01                                                          CC              ⁢                                                          ⁢              02                                                                          CC              ⁢                                                          ⁢              10                                                          CC              ⁢                                                          ⁢              11                                                          CC              ⁢                                                          ⁢              12                                                                          CC              ⁢                                                          ⁢              20                                                          CC              ⁢                                                          ⁢              21                                                          CC              ⁢                                                          ⁢              22                                          )        ⁢          (                                    R                                                G                                                B                              )      
Recently, digital cameras have poor color separation due to increases in pixels of imaging devices. Poor color separation means that in the spectral sensitivity characteristic of an imaging device, spectral sensitivity of R, G, and B is poor, green or blue is mixed in red, and an image may not be expressed in accurate color. For example, it is a phenomenon that when a red subject is photographed by an imaging device having a Bayer array, only red pixels normally should respond but green and blue pixels around the red pixels also respond together. Thus, when an image of an imaging device having poor color separation is processed, color correction is performed to a high degree in order to match the color space of a camera with the color space of an output apparatus.
Performing color correction “to a high degree” means that diagonal elements in a matrix that is a color correction matrix, for example, in a 3×3 matrix, are large. Performing color correction “to a low degree” means that the 3×3 matrix is close to a unit matrix. When color correction is performed to the lowest degree, the color correction matrix is a unit matrix. The color correction matrix is set such that the sum of matrix elements in a row direction becomes 1.CC00+CC01+CC02=1.0CC10+CC11+CC12=1.0CC20+CC21+CC22=1.0
The following color correction matrix is one example where color correction is performed to a high degree. When diagonal elements in the color correction matrix are large, it means that color correction is performed to a high degree.
      (                            Rc                                      Gc                                      Bc                      )    =            (                                    1.8                                              -              0.7                                                          -              0.1                                                                          -              0.4                                            1.8                                              -              0.4                                                                          -              0.1                                                          -              0.7                                            1.8                              )        ⁢          (                                    R                                                G                                                B                              )      
In the case of the existing technology in FIG. 1, the upper limit and the lower limit of RGB data obtained after color correction is performed to a high degree need to be subjected to a clip process such that values of the RGB data fit into a memory area prepared in advance for RGB data. For example, in the case where the memory area for RGB data is an unsigned 12-bit integer, when a value of the RGB data is greater than 4095, the value needs to be changed to 4095, and when a value of the RGB data is less than 0, the value needs to be changed to 0. Due to the clip process, a false outline that does not exist in the actual image may occur. In other words, due to the clip process, an image is provided, which changes sharply at a part where the original image changes smoothly, and hence the image is in a state as if an edge exists therein.
In the existing technology in FIG. 1, it is a problem to detect, as an edge, the false outline that is a kind of noise. Further, when color correction is performed to a high degree for RGB data in a color correction process, noise increases, and noise is mixed in a luminance component at gamma correction. Thus, noise of the luminance Y obtained after the YC conversion increases, which is a problem. As a result of the increase in the noise of the luminance Y, edge detection is easily affected by noise, and an unsmooth edge is generated in a process of emphasizing an edge, which is a problem.
FIGS. 2A and 2B illustrate another existing edge emphasis technology.
A process in FIG. 2A and a process in FIG. 2B cooperate to generate one image. A luminance Y is generated in the process in FIG. 2A, and color differences CbCr are generated in the process in FIG. 2B. In other words, the luminance signal Y is generated in the process in FIG. 2A, while RGB signals are color-corrected at a color correction unit 20, gamma-corrected at a gamma correction unit 21, and YC-converted at a YC conversion unit 22. Edge detection is performed at an edge detection unit 23 for the luminance signal Y, and a signal Ead that has been edge-adjusted at an edge adjustment unit 24 is added to a luminance signal Yn that has been noise-removed at a noise removal unit 25, thereby generating a luminance signal Yee. In the process in FIG. 2A, a unit matrix is used for a matrix operation for color correction. By so doing, color correction is not performed when the luminance signal Y is generated. As a result, an adverse effect due to noise does not occur.
In the process in FIG. 2B, in order to generate the color differences CbCr, color correction is performed to a relatively high degree in the color correction process. In other words, RGB signals that are the same as the RGB signals processed in FIG. 2A are color-corrected at the color correction unit 20 and gamma-corrected at the gamma correction unit 21, and the color-difference signals CbCr are generated at the YC conversion unit 22. Then, the luminance signal Yee obtained in the process in FIG. 2A and the color-difference signals CbCr obtained in the process in FIG. 2B are combined to obtain one image.
In the technology in FIGS. 2A and 2B, an adverse effect on a luminance signal due to noise may be suppressed, but two processes are needed to generate one image.
Further, because the unit matrix is used for the color correction in the process in FIG. 2A, luminance data becomes nonstandard, and color reproducibility is poor. Here, the luminance data becoming nonstandard means that the luminance data does not comply with the standards of the color space of an output apparatus because a color correction process for matching the color space of a camera with the color space of the output apparatus is not performed during a period of generating the luminance data.